Agriculture Reference
In-Depth Information
Apart from systemic poisonings, fungicides as a class have probably
caused disproportionate numbers of irritant injuries to skin and mucous mem-
branes, as well as some dermal sensitizations.
Fungicides may be differentiated according to their mode of action into
protective or surface acting and curative or systemic (internal) acting. The
classification principle used in this text is based on the chemical of the fungi-
cides in order to clearly summarize specific groups or families of compounds.
It will not be possible to discuss all the agents or formulations commercially
available today and, therefore, this overview must be restricted to the most
important groups and their members.
The following discussion considers the mode of action and the toxicity of
the more widely used fungicides and some formulated products.
A.
PROTECTANT OR SURFACE ACTIVE FUNGICIDES
Many commercial fungicides currently in use belong to the class known
as protectant or surface fungicides. These are usually applied to plant foliage
as dusts or sprays. Such materials do not appreciably penetrate the plant sur-
face of cuticle and are not translocated within the plant, whereas the more re-
cent systemic fungicides are absorbed by the plant via the roots, leaves, or
seeds and are translocated within the plant.
Most pathogenic fungi penetrate the cuticle and ramify through the plant
tissue. If one chooses to use a protectant fungicide, it must be applied before
the fungal spores reach the plant if it is to be effective. Only a few fungi are
restricted to the surface of the leaf (e.g., powdery mildews), and in these cases,
protectant fungicides may also possess eradicant action.
For protectant fungicides to be effective, they must satisfy the following
conditions:
1.
They should possess a low phytotoxicity and, therefore, they should not
cause damage the host plant during application.
2.
They should be toxic to the fungal spore or undergo rapid conversion to a
toxic form within the spore before the fungus penetrates the cuticle.
3.
They must be capable of penetrating the spore and reach their site of ac-
tion.
4.
Since most fungicides are applied as foliar sprays, they must be formu-
lated so that when deposited on the plant, they are resistant to the effects
of weathering.
None of the protectant fungicides in use today are completely nonphyto-
toxic. Since fungi and plants both possess the same or similar metabolic or
